First, let me touch on a couple of your listed points.
2- No need to replace em all, but a little piece of mind never hurt. If you can afford it, do all 8.
3- If it currently has red ELC in it, you're probably ok, but if it has anything else, you're going to want to do a full flush, probably a reverse flush as well, and then a fill with some quality EC-1 rated ELC coolant. Be careful, though to not use the nitrite free. It's still up for debate whether our engines need the nitrites, but no one has done a long term test to find out the effects.
4 - If you're going full reliability, I would go stock injectors bought from Ford. If you want a little more power, there are ways with the stock sticks. If you simply must have even more power, go with the 155s.
5- You said you hope this isn't going to be another "Paine"....one of the best things you can do in that regard is research as much as you can before you do anything. On that note, I don't recommend the one at a time method for the studs. If the previous owner was running a tune with 205s, there's a good possibility that you need head gaskets, or that you will very soon. Do the job right. Pull the heads, have em checked/flattened if needed, clean up/flatten the deck, install new HG with the studs. Yes, this can be done with the cab on with a little ingenuity and following in the steps of others who have done it before.
6-If you wind up doing a reverse flush, one step is to do hose direct to the coolant outlet of the oil cooler. You'll probably be able to tell based on the flow rate whether you need a new cooler or not. Mine had water coming out of the block at nearly the same flow rate as the hose. Another I did was clearly less. However, my pre-flush deltas (difference in temperature between the coolant and oil) was a good number...9 degrees I think. Their deltas were 19, which indicates a clogged oil cooler. The flush was an attempt to clear it out, but didn't work.
7- Agreed. If you go stock tune, you may want to consider the FICM
tune from PHP. You can literally add 100 HP and about 125 lb/ft of torque with just that. If you do get a tune AND want the FICM
tune, have a custom tune drawn up which takes that into consideration.
On another note, and only because you said you want reliability, you may want to eventually consider replacing that CAI. The stock filter has been proven to flow enough air for these engines even when they have some major HP upgrades...plus their filtration capability is far superior to those K&N style filters.
Honestly, for the most part, if the previous owner did everything he said, and he took good care of it, then you're probably already heading down the right path. If it were my truck, and I had the money, I would replace the oil in both axles, flush the tranny fluid and upgrade to a 2008-up pan with filter (if you don't have a dealer with the heated flush machine, you can do a drain/fill three times, while running about 100 miles in-between, and that is supposed to be almost as good), check all suspension joints/u-joints and replace/lube. Everyone has a favorite oil or favorite additive, so just be sure it's quality stuff. Me, I like to use Archoil in the engine oil and in the axles. Things run nice and smooth; zero stiction that I can notice. Also, electrical is HUGE in this vehicle! A good electrical system can make or break your truck. Get those batteries tested, and if one of them is close to failing, replace them both....always replace both. If you can, replace them with AGM batteries. They discharge more amps and accept a charge quicker than standard batteries.....plus, they don't corrode the terminals/cables. Another thing is gauges, or at least something to monitor engine vitals. Some use one of the engine tuners for its screen. Some use Torque-Pro on a small tablet. I opted for a gauge cluster to monitor my most important vitals, and use Torque for the rest. However, know that fuel pressure and EGT
cannot be monitored through the computer. You have to install sensors for those, and, unless your monitor has inputs for those (some do), physical gauges for the readouts. Oh yeah...you NEED something to read codes and check certain parameters should anything go wrong. A phone with Torque Pro and FORScan Lite, along with a ELM OBD-II adapter is generally sufficient for most.
Hmmm....trying to think of things off the top of my head. If you still have the plastic CAC
tube going to your intake, replace it with a metal one. Stock boots on the CAC
tubes? Upgrade to quality silicone....I like Riff Raff Diesel's. Pull the connector off your ICP
sensor and inspect for oil. If you do have to replace your oil cooler, be sure to use Ford parts! I bought an aftermarket kit one time by accident (didn't read the listing closely enough), and it wasn't quite the same. It was firmer, less forgiving. I had no choice so I installed it anyway. It was leaking like a sieve within two months. Also, if you're in that far to replace your oil cooler, may as well, replace the HPOP
cover gasket. Those are known to leak....the key is some black RTV along the rear cover. For some reason it tends to sit just a hair lower than the block, making it leak easier. And, of course, while you have the cover off, check to see if they replaced your STC fitting. If not, replace it now. Your HPOP
is the good design, so hopefully that shouldn't need replacing any time soon. You didn't state if the previous owner replaced an other o-rings, such as the stand pipe and dummy plugs, so if you're not sure you may want to plan to do those eventually, along with the nipple cup and injector o-rings.
Really, there's not much else for bulletproofing. Do some searches on here and see what else others have done. For example, I replaced all of my battery cables and upgraded them to 2/0, plus, with my 250 amp alternator, I did what is called the Big 3....improves charging ability significantly over the stock wiring. Oh, the starter cable is still factory...for now. I've installed an oil bypass cooler, have a built-in battery charger so I can just plug it in similar to the block heater (numerous short distance trips just don't allow the batteries to get fully charged again, so the occasional charger session is necessary), and have rebuilt the braking system. There's a lot you can do, and it's only limited by your imagination and your wallet. Your wallet will probably give out before your imagination does. lol
Good luck and keep us informed!!